The present invention relates generally to nuclear reactors, and, more specifically, to a mechanism for refueling a reactor core in a pressure vessel of a boiling water reactor.
A boiling water reactor (BWR) includes a reactor core submerged in water in a closed reactor pressure vessel, with the vessel being disposed in a containment vessel above which is disposed a refueling pool of water. During a refueling cycle, the upper head of the pressure vessel is removed along with internal components of the vessel to provide access to the reactor core therein. Water completely fills the open vessel up to the level of the pool disposed thereabove, and a conventional mobile refueling gantry or platform is movable above the pool for carrying replacement fuel bundles to the reactor core and removing spent fuel bundles therefrom. The platform typically travels longitudinally on rails and has a trolley which moves transversely between the rails, and a telescoping mast with a grapple at the bottom end thereof is carried by the trolley so that fuel bundles may be accurately moved underwater to and from the reactor core.
The core typically includes a two-dimensional horizontal array of square cells in which square fuel bundles are longitudinally inserted and withdrawn therefrom. Since the elevation distance from the refueling platform to the reactor core is typically relatively large, for example about 30 meters, the difficulty and time required to complete the refueling process is relatively high. Since the fuel bundle fits into its mating cell with close tolerances, a significant amount of operator time is required for close-in accurate positioning the fuel bundle therein. And, in the event of any water currents occurring during the refueling operation, the difficulty of accurately inserting a fuel bundle into its mating cell is further increased.
In a typical refueling cycle, about 30 percent of the fuel bundles in the reactor core are replaced. And, a significant portion or all of the remaining fuel is often shuffled or transferred from one position in the core to another for obtaining improved fuel burnup to extend the life thereof. The conventional refueling platform, therefore, must not only remove spent fuel bundles and install replacement fuel bundles, but must also shuffle the remaining fuel bundles between cells to complete the entire refueling operation. Of course, this takes a substantial amount of time which increases costs, as well as increases the amount of radiation exposure time.